What is the safest (and reliable) way to deal with unused input/output pins of serial comm. chips like MAX232, ST3232, etc..

So far i'm familiar with these three options:

  • Input and Output Not connected
  • Tie inputs toghether (ie. MAX232 connect pint 10 to 11, and 8 to 13), output not connected
  • Pull-up/Pull-down resistors to input, output not connected.

I've seen this setups many times and never with a proper explanation as of why it was chosen. Maybe there's another setup i'm not familiar as well.

  • \$\begingroup\$ Datasheet says? \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 18 '17 at 14:40
  • \$\begingroup\$ @IgnacioVazquez-Abrams no indications in datasheet regarding unused inputs... And i've seen many datasheets (i might have missed something, if i have, i'll gladly accept a reference to a datasheet) \$\endgroup\$ – ndelucca Oct 18 '17 at 14:42
  • \$\begingroup\$ I don't know which datasheets you're looking at, but both the TI and Maxim datasheets for the MAX232 are reasonably clear about it. \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 18 '17 at 14:45
  • \$\begingroup\$ @IgnacioVazquez-Abrams Take TI datasheet for example... where does it say anything specific about unused inputs and outputs? It merely explains if they have an internal resistor or how a 0/1 on input translates to the output. I'm asking a question regarding performance of used inputs with different configurations on the unused ones.. I'm looking for an answer from someone who has used this devices for a long time and has seen pros/cons of different configs, or maybe a technical reason... \$\endgroup\$ – ndelucca Oct 18 '17 at 14:58
  • \$\begingroup\$ "Internal pull up resistors on TIN inputs ensures a high input when the line is high impedance." "An open input will result in a high output on ROUT." \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 18 '17 at 14:59

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